Win2003 Mappring drive problem

Hi,

We have a Windows 2003 server which is used as our Data server.

The problem is, our other office that is connected via a VPN tunnel is
unable to map a drive to our Data server, the connection looks like its been
made ie it shows the mapped connection in explorer but the explorer box does
not appear, if you then double click on the map drive it comes back saying
unable to connect.
When I try to map a drive from this Data server to another server on the
domain it does not work.
Our other Office can map drives to our other servers ie RAS, but the problem
is only when mapping to this 2003 server.

The only way around it is for us to reboot the server.

This problem occurs around once a month.

Any help appreciated.